The field of smart materials and intelligent structures has been gradually developing over the past few decades, increasingly enabled by technological advances in the areas of sensors, engineering materials, and actuators. The basis of many actuator and sensor technologies has increasingly been found in emerging “active materials.” Active materials, as a category, are materials that change their shape in response to an external control stimulus, typically a field, such as a thermal, magnetic, or electric field, but also radiation (light) or a changing chemical environment. Materials in this broad category include several classes, often delineated by the stimulus and material type: shape memory alloys, (SMAs), shape memory polymers (SMPs), piezoelectric ceramics, magnetostrictives, and electroactive polymers. Within each of these classes, there are many materials; e.g., within electroactive polymers alone there are a wide variety of low- and high-voltage-activated materials with widely-varying properties, such as ionic-polymer metal composites, conductive polymers, gels, and others.
Additionally, deployable and/or deformable structures have been obtained using active materials incorporated with linear elastic materials. However, a significant change in shape and form of the structural component is hampered by the sub-optimal use of linear elastic materials. Conventional design methodologies using linear elastic materials are an inappropriate choice for large structural shape changes. In one case, the structural components can be designed such that stiffness and strength are low, but the overall reversible deformation is large. In another approach the structure may be designed such that the system exhibits high stiffness, but as a result the reversible deformation capability of the structure is limited. In this case large amounts of energy are required to affect a change in the geometry, along with a significant support structure to maintain the deformation. In addition, high stiffness materials cannot be made to produce significant “Gaussian Curvature” or simultaneous curvature about two orthogonal axes because this requires a change in area in the plane of the deformation. The Poisson effect severely limits the ability of linear elastic materials to change area.
The capability for structural components to achieve large changes in shape would be greatly enabled by materials that can reversibly change their elastic stiffness. By changing elastic stiffness, the energy required for deformation can be decreased. Further advantages would result from a material, which in a lowered stiffness state could undergo large reversible deformation in at least one direction. Given these properties, it would be possible to consider making large changes in the shape of high-stiffness structural components. While this property is currently available using polymers, especially shape memory polymers, these materials used in their bulk form are difficult to control, and building up thick sections for adequate bending stiffness results in slow responding structures.
Current structure design using static component shapes often requires a compromise between various operational conditions rather than optimization over a range of conditions. If an intermittent change in structure is required, for example in the stowing or deploying of a structure, reconfiguring of an aircraft wing from a take-off or landing configuration to a cruising configuration, current solutions require extra components and added complexity is incurred. This problem is not limited to aerospace but is common to a myriad of technologies, such as for example automotive, space, telecommunications, medical, optical, or other technologies where structural or surface reconfiguration is desirable.
Other important areas where a change in stiffness is desirable include storage and deployment of expandable structures. Current methods rely heavily on complicated assemblies of rigid parts that make use of traditional mechanical components (pivots, latches, etc. . . . ). Deployable devices using variable-stiffness structures would enable new designs to be considered, with fewer parts and assembles, thus reducing weight and complexity.
U.S. Pat. No. 6,000,660 by Griffin et al., herein incorporated by reference, describes a variable-stiffness member which changes its stiffness by rotating an elliptical shaft, thereby changing the bending stiffness according to the change in height and width of the elliptical cross section. This concept is limited in the total change in stiffness achievable, and it is not nearly as robust as a material that exhibits intrinsic change in stiffness. In addition, this approach is not applicable to creating stiffness changes or structural shape changes in large planar surface components.
Fibrous elastic memory composite materials utilizing carbon fibers and a shape memory polymer matrix can change their elastic stiffness. Details of this approach are outlined by Campbell and Maji, in the publication “Deployment precision and mechanics of Elastic Memory Composites,” presented at the 44th Structures, Structural Dynamics, and Materials Conference, AIAA, Norfolk, Va., 2003. The limitation of these materials comes from the use of fibers as the reinforcement agent, which undergo micro buckling in order to achieve high compressive strain. The mechanical properties of the composite material once microbuckling has been initiated are significantly reduced as compared to the initial aligned fiber direction. In addition the process of microbuckling can be difficult to control in terms of the direction of microbuckling (i.e. in-plane or out of plane). Another important limitation of these materials in the case of deforming surfaces is a limitation in the amount of area change (simultaneous strain in two orthogonal directions) that can be achieved due to the inextensibility and length of the reinforcing fibers.
Another approach intended to serve the same role as these materials in the specific case of morphing structures are termed compliant structures. These structures employ a specific architecture that tailors deflection along a desired contour when subject to known input forces and deflections. While this approach is advantageous in some applications, it is generally limited in the number of configuration states that can be achieved and optimized. Therefore, it is not as effective when more than one pre-specified range of motion is desired.
In some applications what is needed is a structure capable of changing bending stiffness in response to a control signal such that the stiffness may be decreased, the material reshaped, and the stiffness returned after reshaping. Therefore, a continuing need exists for a cellular structure that allows a change in material stiffness to permit a change in the shape of the cellular structure.